This invention relates to a passive facet tracking optical scanning system employing a rotating polygon mirror and, more particularly, to single element optics for a double bounce passive facet tracking optical scanning system.
FIG. 1 shows a prior art raster output scanning system utilizing a rotating polygon mirror for scanning. A light source 10, such as a laser, emits a coherent light beam 12. The light beam 12 passes through the pre-polygon optics 14 and reflects off the rotating polygon mirror 16 with flat reflecting facets 18. As the mirror 16 rotates, the beam 12 is scanned along a scan line on a photoreceptor 22. The direction along the scan line 20 is the tangential plane and the direction perpendicular to the scan line on the photoreceptor 22 is the sagittal plane.
Optical scanner performance is determined by the physical limitations on the speed at which the mirror is rotated, by the angular deflection of the laser beam achieved by reflection from a facet from the rotating polygon, the number of facets, the size of the facets, and the width of the beam being scanned where it is incident on the rotating mirror.
The beam width impacts the scanning speed because it determines the minimum facet size of a facet on the rotating mirror. A larger facet means a larger rotating polygon and hence larger, more costly motor polygon assemblies with higher power motors and/or slower scanning speeds. Scanning speeds, for a given beam width, can be increased by the use of facet tracking devices because they allow a smaller facet to be used and therefore smaller rotating mirrors which can be rotated faster.
Facet tracking devices are known in the art as in U.S. Pat. No. 3,973,826 by Lobb. Lobb describes a passive facet tracking system utilizing a prescanner which, as it rotates with the scanner, produces a variable deflection in the scanned beam so that during a scan period, the beam moves at the speed of a scanner and in the same direction. The prescanner, by slightly deflecting the beam at the speed of the scanner and in the same direction, maintains the position of the beam centered in the scanning facet. Specifically, a beam which is focused on a prescanner is reflected off the prescanner to a concave mirror which causes the beam to converge but not focus on a facet of the scanner. The prescanner is built using cylindrical or curved facets and the scanner is built using flat facets.
In the Lobb patent, the beam is not collimated at the scanner facet in the tangential plane, thus any variation in radius between the facets will translate into scanning errors on the scan line. In a laser printing application using the optical scanner, these scanning errors show up as pixel placement errors visible on a printed page. When the scanned beam is collimated in the scanned plane at the scanner facet, polygon manufacturing tolerances can be relaxed with resultant cost savings.
In the Lobb patent, the beam is not focused on the scanner facet in the sagittal plane. As a consequence, pyramidal errors in the scanner facet and bearing wobble will result in variable spacing between scan lines. In a laser printing application, these errors show up on the printed page as differences in spacing between the printed lines. Even very small differences are apparent, producing unacceptable output quality. When the beam is focused on the scanner facet in the sagittal plane, the effects of pyramidal errors and bearing wobble may be optically removed by focussing the facet as well as the beam from the facet onto the scan line. Again, this allows polygon manufacturing tolerances to be relaxed with resultant cost savings and no loss in print quality.
U.S. Pat. No. 5,559,627, commonly assigned as the present application and herein incorporated by reference, uses the anamorphic optics of a spherical lens and a retro-reflector to collimate and focus the beam in the planes at the scanning facet so that errors produced by radial and pyramidal variations of the scanning facets may be substantially reduced or easily corrected to provide for improved scanning. U.S. Pat. No. 5,475,524, commonly assigned as the present application and herein incorporated by reference, uses a triplet anamorphic lens and a curved cylindrical mirror to collimate and focus the beam in the planes at the scanning facet.
It is an object of this invention to provide single element optics for a passive facet tracking optical scanning system.